In Huntington disease (HD), the main clinical feature is difficulty with saccade initiation, with prolonged latencies.  Patients frequently use an obligatory blink or head turn to start eye movements. Saccadic latency may be used as a marker of disease progression given its direct correlation with the severity of HD.

Vertical saccades are generally more affected than horizontal, although both may be affected.
Delayed saccadic initiation is most prominent when the saccade is made to command or in anticipation of a predictable target, whereas saccades made to visual stimuli have normal latency. HD patients also have impairment in anti-saccade tasks: when the examiner confronts the patient, showing a finger on either the left or right side and asks the patient to look to the side contralateral to the movement of the examiner’s finger. This may be present during preclinical phases of HD.
Paradoxically, patients often cannot suppress saccades during fixation, with increased visual distractibility.

Fixation: saccadic intrusions may be present.

Smooth pursuit: may be impaired.

Gaze holding and VOR: well preserved.

A patient with moderately severe HD is able to maintain fixation on a visual target. He generates vertical and horizontal saccades with mild delay, however, he recruits head thrusts to generate vertical saccades. He is able to generate saccades to visually directed targets, with some impersistence of fixation in vertical gaze. Pursuit is relatively preserved.

(vv)HD1.mp4(tt)

From: Termsarasab P, Thammongkolchai T, Rucker JC, Frucht SJ. The diagnostic value of saccades in movement disorder patients: a practical guide and review. J Clin Mov Disord. 2015;2:14.

This patient, with milder HD, demonstrates a delay in saccadic initiation, and recruits head thrusts for horizontal saccades.

(vv)HD1 second.mp4(tt)

From: Termsarasab P, Thammongkolchai T, Rucker JC, Frucht SJ. The diagnostic value of saccades in movement disorder patients: a practical guide and review. J Clin Mov Disord. 2015;2:14.